1. Field of the Invention
The present invention relates to an electronic imaging apparatus such as an electronic still camera or a video camera.
2. Description of the Prior Art
It is said that the communication in a society sophisticated in information will find its ultimate form in a mode which utilizes electronic images. Those skilled in the art now anticipate a new communication mode in which images are readily available, freely processable and transmittable. TV monitors permit anyone to reproduce and observe images immediately, and communication equipment such as a telephone network or an image transmitter allows immediate transmission of the images to distant places. Further, computers such as personal computers can perform various modes of processings of images which are fed as inputs. In order to improve an electronic still camera so as to be conveniently usable as a terminal device of these new information media, the camera must be designed compact in dimensions and light in weight, or should desirably be as thin as a memorandum-book for convenient use. The known electronic still cameras have external designs which are rather enlarged mainly due to the sizes of the actuators for floppy discs, circuitry and imaging systems including optical systems (lens systems). Speaking of the recording media, the floppy discs are currently being replaced with IC cards, and it is expected that the circuitry will be made far more compact owing to the rapid progress made in the LSI technology. However, the lens system can hardly be made compact, especially in the thickness thereof, by any of the several methods which are conceivable as extension of the conventional concept. When an electronic still camera system is composed in compliance with the NTSC standard, for example, the image surface thereof is a rectangle having an aspect ratio of 3:4, whereby the image formed by the electronic still camera is picked up with an image sensor having an aspect ratio of 3:4 and reproduced on a monitor having the same aspect ratio of 3:4. Therefore, the imaging lens system to be used with the electronic still camera system must form an image with a sufficient amount of light and good qualities over the entire range of the image sensor having the aspect ratio of 3:4. However, the conventional type of imaging lens system has a nearly circular effective surface and forms an image which is larger than the image sensor so as to cover the diagonal length of the monitor screen, thereby constituting a hindrance to compact design of the electronic still camera.
General lens systems comprise large lens components, especially on the most object side, but have circular sections which are selected from the viewpoint of convenient shaping. In order to solve the above-mentioned problem, it is desirable to cut off the portions which allow transmission of the rays which do not serve for forming an image on the effective surface of the image sensor, or to match the external shapes of lenses with the shape of the effective surface of the image sensor. When lenses are cut so as to have a rectangular external shape matched in aspect ratio with the effective area of the image sensor, for example, a surplus step is necessary for manufacturing but a little effect is obtained for thinning the camera to be composed thereof, and simple matching of the external shapes between lenses and the image sensor is not so desirable.
Furthermore, it is irrefutable that the above-mentioned electronic images are generally lower in quality, especially in resolution, than printed images. The lower resolution is mainly due to too small a number of picture elements which are arranged on the image sensor. On an imaging surface measuring 4.8 mm.times.6.4 mm, for example, 490.times.670 picture elements are currently arranged. Since a limit has been almost reached in the attempt to increase the number of the picture elements by reducing the size of each picture element, another attempt is currently made to maintain S/N ratio at a high level, for example, by providing a micro lens for each picture element. Accordingly, strict restrictions are imposed on the way to arrange picture elements at a density as high as that on the silver halide film 35 mm full size. Another way to arrange picture elements at a higher density lies in forming a larger image of object so as to obtain an enlarged imaging surface, but this way will inevitably enlarge the optical system and is contrary to the days' trend to use compact optical systems.
By the way, the imaging surface of the image sensor is currently designed as a rectangle having the aspect ratio of 3:4 as described above and this ratio is supposed to be lowered by the standard for the future high quality television. The imaging lens systems to be used with the image sensors must assure satisfactory optical performance at least within the internal ranges of the circumscribed circles of the imaging surfaces (rectangles). So far as an imaging surface has a definite area, the circumscribed circle thereof has a longer radius and the imaging lens system to be used therewith is inevitably enlarged as the imaging surface has a lower aspect ratio.
For an optical system having a definite size, on the other hand, a higher aspect ratio of an imaging surface will oblige to use a smaller number of picture elements, thereby degrading optical efficiency. In other words, when the optical system has a definite size, a square imaging surface has the maximum light receiving area and permits arranging the maximum number of picture elements thereon so far as the size of a single picture element remains unchanged. Since the number of picture elements to be arranged in a vertical row is specified by the standard, resolution in the horizontal direction can be enhanced by arranging an excessive number of picture elements in the horizontal direction. When the imaging surface has the square shape, however, the image reproducing screen in compliance with the NTSC standard has the aspect ratio of 3:4 and a screen having a lower aspect ratio is required for the high quality television.